Flow rate sensors are also known as volume sensors. Generally, they take the form of displacement meters. Examples of these are gear sensors, screw spindle meters, oval-wheel meters, cylindrical-piston meters or alternatively measuring turbines or proportioning gear pumps. They are used to measure a volume, a throughflow quantity or the rate at which a medium, here therefore a fluid, passes through the measuring instrument. The fluids may be liquids, pastes or gases.
In practice, flow rate sensors are often not measuring instruments in the narrower sense because the evaluation electronics are not part of the instrument but situated externally. Nevertheless, the term “flow rate measuring instrument” is often used and reference is also made to measuring chambers and measuring mechanism elements etc. The flow rate sensors are frequently also described as volume sensors, throughflow sensors, flow rate measuring instruments etc.
The volume sensors or flow rate sensors merely sense the flow or a volume that has flown through and transmit a signal to the evaluation unit or evaluation electronics, which only then produce a measured value therefrom. The expression “flow rate sensor” is used below. A confusion with specific structural elements in the instrument that are detectors or sensors in the narrower sense is avoided by use of the full designation.
Flow rate sensors or volume sensors in the form of gear sensors are known for example from EP 0 053 575 B1, EP 0 393 294 A1 or DE 40 42 397 C2 as well as EP 0 741 279 B1. They have a housing comprising two halves. In the one housing half, a pair of circular gear wheels are mounted in a freely rotatable manner in a measuring chamber on fixed axles by means of ball bearings and without wall contact. The two gear wheels mesh with one another. The medium, the displaced volume or flow rate of which is to be determined, is fed through a first bore to the two gear wheels, namely into the region where these gear wheels mesh with one another. The medium therefore passes into the chambers that are mutually formed in the tooth spaces of the two gear wheels. As a result of the following flow of the medium, the quantities situated in the chambers of the gear wheel are conveyed from the inlet side to the outlet side and by means of the movement of the teeth then set the gear wheels in rotation. The two gear wheels in said case rotate in opposite directions. At the other side of the gear wheels in flow direction downstream of the meshing region, the medium is discharged through a second bore.
The other housing half serves as a top cover for the region of the measuring chamber having the two gear wheels plus the medium flowing here. It therefore tightly closes the measuring chamber and prevents fluid streams from being able to pass through the measuring chamber outside of the meshing region of the gear wheels. The two housing halves lie one flat on top of the other and between them lies a, so to speak, virtual parting plane.
Permanent magnets such as for example in DE 40 42 397 C2 or carrier frequency sensors as in EP 0 741 279 B1 are provided in the housing adjacent to the meshing region of the gear wheels and build up an electromagnetic field. This field is varied by the teeth of the gear wheels and/or by the movement thereof.
The second housing half in the known arrangements mentioned above moreover accommodates a magnetoresistive differential sensor. The magnetoresistive differential sensor senses the variations of the fields caused by the movement of the teeth of the rotating gear wheels.
The sensor in these known instruments is separated from the medium or fluid to be measured by a non-magnetic insert, which protects the sensor in particular from the physical and chemical stresses imposed by the medium. The medium may not only have a very different temperature or consistency but may also be chemically aggressive.
This protection leads disadvantageously to a spacing of the sensor from the teeth of the rotating gear wheels that makes measurement more difficult and limits the accuracy and reliability of measurement. By means of suitable pole pins or other measures, the movement of each tooth flank during rotation of the associated gear wheel relative to the magnetoresistive differential sensor is then detected and communicated externally to suitable evaluation units.
Particularly with larger piece numbers of flow rate sensors, the economic aspect gains in importance. Nevertheless, even with larger piece numbers the accuracy and precision remains important. In many cases, the flow rate of a fluid is also adjusted in closed-loop control circuits depending on the measurement.
It would therefore be desirable to carry out as precise as possible a measurement of the quantity and/or rate of flow of a fluid by means of flow rate sensors that entail the least possible outlay.
The object of the invention is therefore to propose flow rate sensors that combine a particularly economical design with nevertheless accurate measurement results.